KrF excimer lasers are the state of the art light source for integrated circuit lithography. One such laser is described in U.S. Pat. No. 4,959,840 issued Sep. 25, 1990. The lasers operate at wavelengths of about 248 nm. With the KrF laser integrated circuits with dimensions as small as 180 nm can be produced. Finer dimensions can be provided with ArF lasers which operate at about 193 nm or F2 lasers which operate at about 157 nm. These lasers, the KrF laser, the ArF laser and the F2 lasers, are very similar, in fact the same basic equipment used to make a KrF laser can be used to produce an ArF laser or an F2 laser merely by changing the gas concentration, increasing the discharge voltage and modifying the controls and instrumentation to accommodate the slightly different wavelength.
A typical prior-art KrF excimer laser used in the production of integrated circuits is depicted in FIGS. 1, 2 and 3. A cross section of the laser chamber of this prior art laser is shown in FIG. 3. As shown in FIG. 2, pulse power system 2 powered by high voltage power supply 3 provides electrical pulses to electrodes 6 located in a discharge chamber 8. Typical state-of-the art lithography lasers are operated at a pulse rate of about 1000 to 2000 Hz with pulse energies of about 10 mJ per pulse. The laser gas (for a KrF laser, about 0.1% fluorine, 1.3% krypton and the rest neon which functions as a buffer gas) at about 3 atmospheres is circulated through the space between the electrodes at velocities of about 1,000 to 2,000 cm per second. This is done with tangential blower 10 located in the laser discharge chamber. The laser gases are cooled with a heat exchanger 11 also located in the chamber and a cold plate (not shown) mounted on the outside of the chamber. The natural bandwidth of the excimer lasers is narrowed by line narrowing module 18 (sometimes referred to as a line narrowing package or LNP). Commercial excimer laser systems are typically comprised of several modules that may be replaced quickly without disturbing the rest of the system. Principal modules include:
Laser Chamber Module,
High voltage power supply module,
High voltage compression head module,
Commutator module,
Output Coupler Module,
Line Narrowing Module,
Wavemeter Module,
Computer Control Module,
Gas Control Module,
Cooling Water Module
Electrodes 6 consist of cathode 6A and anode 6B. Anode 6B is supported in this prior art embodiment by anode support bar 44 which is shown in cross section in FIG. 3. Flow is counter-clockwise in this view. One corner and one edge of anode support bar 44 serves as a guide vane to force air from blower 10 to flow between electrodes 6A and 6B. Other guide vanes in this prior art laser are shown at 46, 48 and 50. Perforated current return plate 52 helps ground anode 6B to the metal structure of chamber 8. The plate is perforated with large holes (not shown in FIG. 3) located in the laser gas flow path so that the current return plate does not substantially affect the gas flow. A peaking capacitor bank comprised of an array of individual capacitors 19 is charged prior to each pulse by pulse power system 2. During the voltage buildup on the peaking capacitor, one or two preionizers 56 weakly ionize the lasing gas between electrodes 6A and 6B and as the charge on capacitors 19 reaches about 16,000 volts, a discharge across the electrode is generated producing the excimer laser pulse. Following each pulse, the gas flow between the electrodes of about 1 to 2 cm per millisecond, created by blower 10, is sufficient to provide fresh laser gas between the electrodes in time for the next pulse occurring one millisecond later.
In a typical lithography excimer laser, a feedback control system measures the output laser energy of each pulse, determines the degree of deviation from a desired pulse energy, and then sends a signal to a controller to adjust the power supply voltage so that energy of the subsequent pulse is close to the desired energy. These excimer lasers are typically required to operate continuously 24 hours per day, 7 days per week for several months, with only short outages for scheduled maintenance.
A well-known technique for reducing the band-width of gas discharge laser systems (including excimer laser systems) involves the injection of a narrow band xe2x80x9cseedxe2x80x9d beam into a gain medium. In one such system, a laser called the xe2x80x9cseed laserxe2x80x9d or xe2x80x9cmaster oscillatorxe2x80x9d is designed to provide a very narrow laser band beam and that laser beam is used as a seed beam in a second laser. If the second laser functions as a power amplifier, the system is typically referred to as a master oscillator, power amplifier (MOPA) system. If the second laser itself has a resonance cavity, the system is usually referred to as an injection seeded oscillator (ISO) and the seed laser is usually called the master oscillator and the downstream laser is usually called the power oscillator.
A typical KrF laser has a natural bandwidth of about 300 pm (FWHM) centered at about 248 nm and for lithography use, it is typically line narrowed to about 0.6 pm. ArF lasers have a natural bandwidth of about 500 centered at about 193 nm and is typically line narrowed to about 0.5 pm. These lasers can be relatively easily tuned over a large portion of their natural bandwidth using the line narrowing module 18 shown in FIG. 2. F2 lasers typically produce laser beams with most of its energy in two narrow lines centered at about 157.63 nm and 157.52 nm. Often, the less intense of these two lines (i.e., the 157.52 nm line) is suppressed and the laser is forced to operate at the 157.63 nm line. The natural bandwidth of the 157.63 nm line is pressure dependant and varies from about 0.6 to 1.2 pm. An F2 laser with a bandwidth in this range can be used with lithography devices utilizing a catadiophic lens design utilizing both refractive and reflective optical elements, but for an all-refractive lens design the laser beam should have a bandwidth of about 0.1 pm to produce desired results.
There are many optical filters for selecting out narrow ranges of light in a beam. One such filter is a monochromator in which light passing through a first slit is collimated with a lens, dispersed spectrally with a dispersing element such as a prism or grating and the dispensed light is then focused to a focal plane with a selected spectral range collected through a slit located at the local plane.
What is needed is an improved narrow band F2 laser system.
The present invention provides a narrow band F2 laser system useful for integrated circuit lithography. An output beam from a first F2 laser gain medium is filtered with a pre-gain filter to produce a seed beam having a bandwidth of about 0.1 pm or less. The seed beam is amplified in a power gain stage which includes a second F2 laser gain medium. The output beam of the system is a pulsed laser beam with a full width half maximum band width of about 0.1 pm or less with pulse energy in excess of about 5 mJ. In a preferred embodiment the pre-gain filter includes a wavelength monitor which permits feedback control over the centerline wavelength so that the pre-gain filter optics can be adjusted to ensure that the desired bandwidth range is injected into the power gain stage.
The present invention provides a major system-level advantage over attempting to line-narrow the master oscillator in a conventional way. In conventional line-narrowing, the dispersive optics (etalons, gratings, etc.) are inserted into the optical resonator of the first laser. As a result, if the wavelength that the line-narrowing optics are selecting is incorrect, the master oscillator will not even lase. For the F2 laser, with approximately a 1 pm wide gain, the line narrowing optics must be tuned to within xc2x10.5 pm even before the laser is fired. This places an extremely difficult requirement on the stability of the line-narrowing optics, especially after long dormant periods with the laser off or in standby mode. In general, one cannot depend on the laser producing any significant light after long off periods. To accomodate this, one must resort to xe2x80x9cblind searchxe2x80x9d methods to try to re-aquire lasing. In contrast, the free-running master oscillator of the present invention will always lase, even if the injection spectral filter is out of tune. This gives the opportunity to correct the tuning of the injection filter.